The present invention relates to a process for separating mixtures of materials having different boiling points. More particularly, it is a process for separating a technical product containing both a desired product and unwanted byproducts. This process is particularly effective for separating isomers, especially isomeric mixtures of amines.
Many production processes yield mixtures of materials containing high boiling residues and low-boiling isomers that need to be separated in order to recover the desired product. One example of such a process is the process for producing aromatic amines such as toluene diamine (xe2x80x9cTDAxe2x80x9d). Processes for the production of aromatic amines are known. One such process is disclosed in U.S. Pat. No. 5,728,880. In the known processes for the production of aromatic amines, the hydrogenation product is generally an isomeric mixture of amines from which the desired isomer or mixture of isomers needs to be separated.
Several techniques for achieving such separation are known. In the technique disclosed in U.S. Pat. No. 5,728,880, for example, any solvent present or water generated by the hydrogenation reaction is removed from the reaction mixture. The resultant mixture is then distilled to separate the low boiling amine isomers from the high boiling by-product materials present. The high boiling by-product materials and some of the meta-isomer remaining in the distillation residue are then concentrated and mixed with some of the ortho-isomer. This mixture is then distilled to remove the meta- and ortho-isomers which are subsequently returned to the TDA isomer distillation column. One advantage of this patented technique is the relatively low amount of additional energy input needed to separate the high boiling component(s). However, the amount of m-TDA recovered is less than optimum. To optimize recovery of the meta isomer, excessive amounts of the ortho isomer must be used. However, use of such large amounts of the ortho isomer causes a high load on the TDA isomer distillation column.
U.S. Pat. No. 5,684,180 also discloses a process for the separation and purification of aromatic polyamine mixtures. In the process described in U.S. Pat. No. 5,684,180, the polyamine-containing reaction mixture is mixed with a two-phase system in a first extraction stage that operates on a counter-current principle. Multi-stage distillation is subsequently used to further separate the components of the polyamine mixture. This patented process is commercially undesirable because it requires a high amount of energy and a costly distillation unit to separate the amine product from the extraction medium.
U.S. Pat. No. 4,480,129 discloses another process for separating isomers, specifically isomers of toluidine. In this disclosed process, the isomer mixture is contacted with an adsorbent satisfying specified compositional requirements to selectively adsorb the p-toluidine. This patented process would not be useful for separating mixtures of materials other than toluidine because high boiling by-products would be expected to foul the adsorbent taught to be necessary to achieve the desired separation. Handling of the solids generated in the course of this process makes commercial use of the process difficult.
It is an object of the present invention to provide a process and an apparatus useful for separating mixtures of materials having different boiling points.
It is also an object of the present invention to provide a process and apparatus useful for separating mixtures of amines, particularly mixtures containing different isomers of an amine, which process and apparatus do not require as much expensive equipment and energy as the known prior art processes and apparatus.
It is another object of the present invention to provide a process and apparatus useful for separating mixtures, particularly isomer mixtures of amines, in which the amount of the desired isomer(s) recovered is significantly greater than the amounts generally recovered by known prior art separation processes and apparatus.
It is a further object of the present invention to provide a process and apparatus useful for separating a desired amine isomer or isomer mixture from an isomeric mixture in which the amount of the desired isomer lost is substantially less than that lost in prior art processes and apparatus.
It is an additional object of the present invention to provide a flexible process and apparatus for separating amine mixtures.
These and other objects which will be apparent to those skilled in the art are accomplished by treating a mixture of materials having different boiling points to separate that mixture into fractions having different boiling points. The fraction containing the desired product is fed to a stripping column in which it is stripped using the vapors of a lower boiling fraction. The desired isomer or isomer mixture in which little or no unwanted isomer or by-products are present is recovered.